Catalysts can be used to change the rates and control the yields of chemical reactions to increase the amounts of desirable products from these reactions and reduce the amounts of undesirable ones. For example, cerium(IV) oxide (CeO2) (also referred to as ceria) can be used in catalytic converters for reducing carbon monoxide emissions in exhaust gases from motor vehicles. Cerium exists in both trivalent state (+3) and tetravalent (+4) state and may switch between the two in reduction and oxidation reactions. Cerium(IV) oxide can be reduced by carbon monoxide to cerium(III) oxide:2CeO2+CO→Ce2O3+CO2,and cerium(III) oxide can be oxidized to cerium(IV) oxide:2Ce2O3+O2→4CeO2.Cerium oxide is also useful in three-way catalysis, petroleum cracking, water gas shift reaction, fuel cells, and synthetic organic chemistry. It has been proposed that the catalytic traits of cerium oxide can be attributed to two properties of ceria: (i) the high mobility and storage capacity of oxygen within the lattice, and (ii) the ease with which cerium changes between Ce3+ and Ce4+ states to allow the high oxygen mobility within the oxide lattice. These properties, combined with the abundance of cerium on earth, make ceria a low-cost and effective alternative to noble metal catalysts.